Toward an animal model of the human tear film: biochemical comparison of the mouse, canine, rabbit, and human meibomian lipidomes

The effects of age and dysfunction on meibomian gland population dynamics

PURPOSE

While meibomian gland dysfunction (MGD) is widely recognized as a major cause of evaporative dry eye disease, little is known about normal gland differentiation and lipid synthesis or the mechanism underlying gland atrophy and abnormal lipid secretion. The purpose of this study was to use single-cell and spatial transcriptomics to probe changes in cell composition, differentiation, and gene expression associated with two murine models of MGD: age-related gland atrophy in wild-type mice and altered meibum quality in acyl-CoA wax alcohol acyltransferase 2 (Awat2) knockout (KO) mice.

METHODS

Young (6 month) and old (22 month) wild type, C57Bl/6 mice and young (3 month) and old (13 month) Awat2 KO mice were used in these studies. For single-cell analysis, the tarsal plate was dissected from the upper and lower eyelids, and single cells isolated and submitted to the UCI Genomic Core, while for the spatial analysis frozen tissue sections were shipped to Resolve Biosciences on dry ice and sections probed in duplicate using a meibomian gland specific, 100 gene Molecular Chartography panel.

RESULTS

Analysis of gene expression patterns identified the stratified expression of lipogenic genes during meibocyte differentiation, which may control the progressive synthesis of meibum lipids; an age-related decrease in meibocytes; and increased immune cell infiltration. Additionally, we detected unique immune cell populations in the Awat2 KO mouse suggesting activation of psoriasis-like, inflammatory pathways perhaps caused by ductal dilation and hyperplasia.

CONCLUSION

Together these findings support novel mechanism controlling gland function and dysfunction.

Safety and Efficacy of Dry Eye Intelligent Therapeutic Device in the Treatment of Meibomian Gland Dysfunction in Rabbits

PURPOSE

To assess the safety and efficacy of the dry eye intelligent therapeutic device in rabbits with meibomian gland dysfunction.

METHODS

The meibomian gland dysfunction-afflicted rabbits were subjected to treatment using the dry eye intelligent therapeutic device. Various parameters, including eyelid margin, meibomian gland opening, redness, meibomian gland area, keratoconjunctival fluorescence staining, and intraocular pressure, were examined and analyzed using an ocular surface comprehensive examination instrument, slit lamp, and tonometer at corresponding times points. Hematoxylin and eosin staining was performed to examine the mucosal epithelium and meibomian gland.

RESULTS

In this study, eyelid margin congestion and meibomian gland opening obstruction were significantly improved after 3 weeks and 4 weeks of treatment, respectively (p < .01, p < .05). The treatment group showed a significant increase in tear meniscus height after 2 weeks, 3 weeks and 4 weeks of treatment (p < .001, p < .01, p < .05). No significant changes were noted in meibomian gland area, redness, intraocular pressure, and keratoconjunctival fluorescence staining of rabbits before and after treatment. Hematoxylin and eosin staining revealed a complete structure of mucosal epithelium and meibomian gland in the treatment group and that the expansion of the blocked meibomian gland duct was reduced.

CONCLUSION

The utilization of the dry eye intelligent therapeutic device in treating meibomian gland dysfunction-afflicted rabbits exhibits potential promising safety, efficacy, and overall benefits, thereby offering a novel alternative for managing meibomian gland dysfunction patients in clinical settings.

Assessment of lipid layer patterns in domestic dogs and rabbits: an observational study

Background

Maintaining a stable tear film is crucial for having healthy human and animal vision. Animals are expected to have thicker lipid layers than humans due to living in high-temperature and humid environments.

Aim

The study aimed to evaluate the lipid layer patterns (LLPs) in Arabian dogs and rabbits using a non-invasive, practical, and easy-to-use device and compare them to humans with healthy eyes.

Methods

The study included 75 domestic Arabian dogs (42 males and 33 females; mean ± SD = 6.1 ± 12.7 months) and 75 rabbits (37 males and 38 females; mean ± SD = 3.1 ± 3.4 months). In addition, 75 individuals with healthy eyes (39 males and 36 females; mean ± SD = 25.7 ± 5.0 years) were included for comparison. EASYTEAR View+ assessed the LLP in each animal's and individual's right eye.

Results

The median LLP grades significantly differed between dogs and humans (Mann-Whitney U test, p < 0.001). Similarly, the LLP grades differed significantly between rabbits and humans (Mann-Whitney U test, p < 0.001). No significant difference (Mann-Whitney U test) in the LLP grades between dogs and rabbits was found. The analysis indicated that most dogs had either an A (34.7%) or a B grade (37.3%). Similarly, rabbits had predominantly A or 1 (46.7%) and B (30.7%) grades. On the other hand, humans had predominantly D (53.3%) and E (30.7%) grades.

Conclusion

The EASYTEAR View+ has been employed to assess LLP in dogs and rabbits, and the measurements were compared to those of humans with normal ocular health. Dogs and rabbits have thinner lipid layers than healthy humans.

Robust and Objective Evaluation of Superficial Punctate Keratopathy in a Murine Dry Eye Model

Purpose

To establish a robust and objective method to evaluate (SPK) superficial punctate keratopathy in a murine dry eye model by developing a reliable photographic system.

Design

Experimental study.

Subjects

A murine dry eye model was generated by exorbital lacrimal gland excision. Sham-operated mice were used as healthy controls. For the sham operation, an incision was made without touching the gland.

Methods

A photographic system was constructed, consisting of an LED lamp and a digital camera fitted with a zoom lens and sharp cut filter. SPK was detected by applying fluorescein solution. To validate the system, SPK was compared between dry eye mice and healthy control mice, and diquafosol (DIQUAS ophthalmic solution 3%; Santen Pharmaceutical Co., Ltd.) or cyclosporine (PAPILOCK Mini ophthalmic solution 0.1%; Santen Pharmaceutical Co., Ltd.) was used to dry eye mice.

Main Outcome Measures

SPK was evaluated using the parameters of fluorescence score and fluorescein-stained area.

Results

The photographs clearly indicated SPK in dry eye mice. A fluorescence score of 0 to 9 could be easily assessed, and the fluorescein-stained area was quantifiable. The fluorescein-stained area correlated with fluorescence score (correlation coefficient: 0.98), with good interobserver reliability (intraclass correlation coefficient: 0.999). The fluorescein-stained area increased significantly in dry eye mice compared with that of healthy controls (P < 0.0001). Both types of therapeutic eye drops decreased the fluorescein-stained area relative to saline-treated mice (P < 0.05 in diquafosol vs. saline; P < 0.01 in cyclosporine vs. saline).

Conclusions

This newly developed system is a robust alternative for quantitative evaluation of SPK in a murine dry eye model.

Financial Disclosures

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

A Comprehensive Assessment of Tear-Film-Oriented Diagnosis (TFOD) in a Dacryoadenectomy Dry Eye Model

Tear film instability is a major cause of dry eye disease. In order to treat patients with short tear film breakup time (TBUT)-type dry eye, the development of tear film stabilizing agents is essential. However, the lack of an appropriate animal model of tear film instability has made drug development difficult. Although rabbit dry eye models have been reported in the past, there are only a few reports that focus on tear film instability. Herein, we assessed the tear film stability of a rabbit dry eye model induced by dacryoadenectomy. A clinical evaluation of the ocular surface, interferometry, and histological assessments of the cornea and conjunctiva were performed. Following the removal of the lacrimal glands, TBUT was shortened significantly, with dimple and random breakup patterns prominently observed. Furthermore, the blink rate in this model increased after dacryoadenectomy, suggesting that this model partially captured the phenotypes of human short TBUT-type dry eye and may be useful as an animal model for investigating potential drug candidates.

Full thickness 3D in vitro conjunctiva model enables goblet cell differentiation

In vitro culture and generation of highly specialized goblet cells is still a major challenge in conjunctival 3D in vitro equivalents. A model comprising all physiological factors, including mucus-secreting goblet cells has the potential to act as a new platform for studies on conjunctival diseases. We isolated primary conjunctival epithelial cells and fibroblasts from human biopsies. 3D models were generated from either epithelial layers or a combination of those with a connective tissue equivalent. Epithelial models were investigated for marker expression and barrier function. Full-thickness models were analyzed for goblet cell morphology and marker expression via immunofluorescence and quantitative real-time PCR. Simple epithelial models cultured at the air-liquid interface showed stratified multi-layer epithelia with pathologic keratinization and without goblet cell formation. The combination with a connective tissue equivalent to generate a full-thickness model led to the formation of a non-keratinized stratified multi-layer epithelium and induced goblet cell differentiation. In our model, a high resemblance to natural conjunctiva was achieved by the combination of conjunctival epithelial cells with fibroblasts embedded in a collagen-hydrogel as connective tissue equivalent. In the future, our conjunctival in vitro equivalent enables the investigation of goblet cell differentiation, conjunctival pathologies as well as drug testing.

Meibomian gland stem/progenitor cells: The hunt for gland renewal

Meibomian glands (MGs) secrete lipid (meibum) onto the ocular surface to form the outermost layer of the tear film. Proper meibum secretion is essential for stabilizing the tear film, reducing aqueous tear evaporation, and maintaining the homeostasis of the ocular surface. Atrophy of MG as occurs with aging, leads to reduction of meibum secretion, loss of ocular surface homeostasis and evaporative dry eye disease (EDED). Since MGs are holocrine glands, secretion of meibum requires continuous self-renewal of lipid-secreting acinar meibocytes by stem/progenitor cells, whose proliferative potential is dramatically reduced with age leading to MG atrophy and an age-related meibomian gland dysfunction (ARMGD). Understanding the cellular and molecular mechanisms regulating meibocyte stem/progenitor cell maintenance and renewal may provide novel approaches to regenerating MG and treating EDED. Towards that end, recent label retaining cell and lineage-tracing experiments as well as knock-out transgenic mouse studies have begun to identify the location and identities of meibocyte progenitor cells and potential growth and transcription factors that may regulate meibocyte renewal. In addition, recent reports have shown that ARMGD may be reversed by novel therapeutics in mice. Herein, we discuss our current understanding of meibocyte stem/progenitor cells and the hunt for gland renewal.

Meibomian gland development: Where, when and how?

The Meibomian gland (MG) is an indispensable adnexal structure of eye that produces meibum, an important defensive component for maintaining ocular homeostasis. Normal development and maintenance of the MGs is required for ocular health since atrophic MGs and disturbances in composition and/or secretion of meibum result in major ocular pathologies, collectively termed as Meibomian gland dysfunction (MGD). Currently available therapies for MGD merely provide symptomatic relief and do not treat the underlying deficiency of the MGs. Hence, a thorough understanding of the timeline of MG development, maturation and aging is required for regenerative purposes along with signaling molecules & pathways controlling proper differentiation of MG lineage in mammalian eye. Understanding the factors that contribute to the development of MGs, developmental abnormalities of MGs, and changes in the quality & quantity of meibum with developing phases of MGs are essential for developing potential treatments for MGD. In this review, we compiled a timeline of events and the factors involved in the structural and functional development of MGs and the associated developmental defects of MGs during development, maturation and aging.

Sdr16c5 and Sdr16c6 control a dormant pathway at a bifurcation point between meibogenesis and sebogenesis

Genes Sdr16c5 and Sdr16c6 encode proteins that belong to a superfamily of short-chain dehydrogenases/reductases (SDR16C5 and SDR16C6). Simultaneous inactivation of these genes in double-KO (DKO) mice was previously shown to result in a marked enlargement of the mouse Meibomian glands (MGs) and sebaceous glands, respectively. However, the exact roles of SDRs in physiology and biochemistry of MGs and sebaceous glands have not been established yet. Therefore, we characterized, for the first time, meibum and sebum of Sdr16c5/Sdr16c6-null (DKO) mice using high-resolution MS and LC. In this study, we demonstrated that the mutation upregulated the overall production of MG secretions (also known as meibogenesis) and noticeably altered their lipidomic profile, but had a more subtle effect on sebogenesis. The major changes in meibum of DKO mice included abnormal accumulation of shorter chain, sebaceous-type cholesteryl esters and wax esters (WEs), and a marked increase in the biosynthesis of monounsaturated and diunsaturated Meibomian-type WEs. Importantly, the MGs of DKO mice maintained their ability to produce typical extremely long chain Meibomian-type lipids at seemingly normal levels. These observations indicated preferential activation of a previously dormant biosynthetic pathway that produce shorter chain, and more unsaturated, sebaceous-type WEs in the MGs of DKO mice, without altering the elongation patterns of their extremely long chain Meibomian-type counterparts. We conclude that the Sdr16c5/Sdr16c6 pair may control a point of bifurcation in one of the meibogenesis subpathways at which biosynthesis of lipids can be redirected toward either abnormal sebaceous-type lipidome or normal Meibomian-type lipidome in WT mice.

Probing dietary triacylglycerol metabolism and meibogenesis in mice: A stable isotope-labeled tracer LC-MS/MS study

Exocrine Meibomian glands (MGs) play a central role in the ocular physiology and biochemistry by producing in situ and, mostly, de novo, a secretion (meibum), which is composed of a complex mixture of homologous lipids of various classes, in a metabolic pathway termed meibogenesis. Recent in vivo experiments with a number of mouse models demonstrated that inactivation of any of the major genes of meibogenesis led to alterations in the lipid composition of meibum and severe ocular and MG abnormalities that replicated various human ocular pathologies. However, the role of dietary lipids in meibogenesis, and in the onset and/or alleviation of these diseases, remains controversial. To uncover the role of dietary lipids, the metabolic transformations of a dietary lipid tracer - stable isotope-labeled glyceryl tri(oleate-1,2,3,7,8-¹³C₅) (¹³C₁₅-TO) - were investigated using LC-high-resolution TOF-MS/MS. We demonstrated that major metabolic transformations of the tracer occurred in the stomach and small intestines where ¹³C₁₅-TO underwent immediate and extensive transesterification into ¹³C₅- and ¹³C₁₀-substituted triacylglycerols of various lengths, giving a mixture of ¹³C-labeled compounds that remain virtually unchanged in the mouse plasma, liver, and white adipose tissue, but were almost undetectable in the feces. Importantly, the tracer and its metabolites were virtually undetectable in MGs, even after 4 weeks of daily supplementation. Notably, unbiased Principal Component Analysis of the data revealed no measurable changes in the overall chemical composition of meibum after the treatment, which implies no direct effect of dietary triacylglycerols on meibogenesis, and left their systemic effects as the most likely mechanism.

A comparative review of evaporative dry eye disease and meibomian gland dysfunction in dogs and humans

Dry eye disease is a complex ophthalmic disorder that consists of two main subtypes, aqueous deficient dry eye (ADDE) and evaporative dry eye disease (EDED). Due to the complex underlying physiology, human dry eye disease can be difficult to model in laboratory animal species. Thus, the identification and characterization of a spontaneous large animal model of dry eye disease is desirable. Dogs have been described as an ideal spontaneous model of ADDE due to the similar pathophysiology between dogs and humans. Recently, EDED and meibomian gland dysfunction (MGD) have been increasingly recognized and reported in dogs. These reports on EDED and MGD in dogs have identified similarities in pathophysiology, clinical presentations, and diagnostic parameters to humans with the comparable disorders. Additionally, the tests that are used to diagnose EDED and MGD in humans are more easily applicable to dogs than to laboratory species due to the comparable globe sizes between dogs and humans. The reported response of dogs to EDED and MGD therapies are similar to humans, suggesting that they would be a valuable preclinical model for the development of additional therapeutics. Further research and clinical awareness of EDED and MGD in dogs would increase their ability to be utilized as a preclinical model, improving the positive predictive value of therapeutics for EDED and MGD in both humans and dogs.

Effect of Age and Viscosity on Duration of Corneal Anesthesia with Topical 2% Lidocaine Administration

Purpose: Formulation viscosity and patient-specific parameters such as age are important considerations in achieving patient comfort for prolonged anterior segment surgical procedures. In this study, we report pharmacodynamic and pharmacokinetic parameters of topical 2% lidocaine anesthetic decay based on formulation viscosity and subject age. Methods: Extemporaneous 2% lidocaine solution was compounded with varying percentages of carboxymethylcellulose (CMC) to adjust product viscosity. Juvenile and adult New Zealand White rabbits were utilized as a model for lidocaine-induced corneal anesthesia analysis. Following application of 20 μL in 1 eye of each animal, corneal sensitivity was measured using a Cochet-Bonnet esthesiometer at baseline and at 1-min intervals until recovery to baseline. Subsequent to washout period, the experiment was repeated for 3 replicate experiments. Results: A one-phase exponential decay model was utilized to describe rate of anesthesia decay. Bioavailability increased in a manner disproportionate to both tear film concentration and solution viscosity. In adult animals, half-life of anesthetic decay was found to range from 6.03 min with 2% lidocaine in 0.5% CMC to 9.45 min with 2% lidocaine in 1.5% CMC. In juveniles, half-life was found to be 4.46 and 3.58 min for 2% lidocaine in 1.5% CMC and commercial 2% lidocaine gel, respectively. Conclusions: Decay parameters of lidocaine-induced corneal anesthesia appear disparate from viscosity. It is postulated that viscosity-related increase in corneal contact time through reduced drainage plays a critical role in increasing bioavailability of topical anesthetics in our experimental findings, although nonlinear in character. Age is found to be an important mediator of lidocaine-induced corneal anesthesia.

Dynamic Changes in the Gene Expression Patterns and Lipid Profiles in the Developing and Maturing Meibomian Glands

Meibomian glands (MGs) and their holocrine secretion-meibum-play crucial roles in the physiology of the eye, providing protection from environmental factors and desiccation, among other functions. Importantly, aging was implicated in the deterioration of the morphology and functions of MGs, and the quantity and quality of meibum they produce, leading to a loss of its protective properties, while the meibum of young individuals and experimental animals provide ample protection to the eye. Currently, the molecular mechanisms of meibum biosynthesis (termed meibogenesis) are not fully understood. To characterize the physiological changes in developing and maturing MGs, we studied the lipidomes and transcriptomes of mouse MGs ranging from newborns to adults. The results revealed a gradual increase in the critical genes of meibogenesis (such as Elovl3, Elovl4, Awat2, and Soat1, among others) that positively correlated with the biosynthesis of their respective lipid products. The MG transcriptomes of young and adult mice were also analyzed using single-cell RNA sequencing. These experiments revealed the existence of multiple unique populations of MG cells (meibocytes, epithelial cells, and others) with specific combinations of genes that encode meibogenesis-related proteins, and identified clusters and subclusters of cells that were tentatively classified as meibocytes at different stages of differentiation/maturation, or their progenitor cells. A hypothesis was formulated that these cells may produce different types of lipids, and contribute differentially to the Meibomian lipidome.

Safety and efficacy of a low-level radiofrequency thermal treatment in an animal model of obstructive meibomian gland dysfunction

This study aimed to evaluate the safety and efficacy of a low-level radiofrequency thermal treatment in an obstructive MGD rabbit model. Meibomian gland orifices of the central two-thirds of the upper and lower eyelid margins were coagulated twice at 2-week intervals using a 5-MHz high-frequency electrosurgical unit. Sixteen eyes of eight rabbits were treated with one session of radiofrequency thermal treatment (radiofrequency group) and eight eyes of four rabbits were followed up without treatment (control group). Lid margin abnormality and corneal staining scores, histologic examination of the eyelids and meibombian gland, and meibography imaging were evaluated just before and 4 weeks after meibomian gland orifice closure and 4 weeks after radiofrequency thermal treatment. Lid margin abnormality score improved significantly for the upper and lower eyelids after radiofrequency thermal treatment (P < 0.001 for both eyelids). Corneal staining score remained unchanged in the radiofrequency group; however, the control group saw an increase at final follow-up. There was a significant improvement to almost baseline levels in the mean area of secretory acini in the radiofrequency group (P = 0.004). Additionally, meibography indicated an improvement in meibomian gland loss rate in the radiofrequency group. Low-level radiofrequency thermal treatment heating the inner and outer eyelid surfaces is safe and effective to treat obstructive MGD in a rabbit animal model of MGD.

Novel Lipids of the Rabbit Harderian Gland Improve Tear Stability in an Animal Model of Dry Eye Disease

Purpose: Instability of the tear film leads to evaporative dry eye disease (EDED), but the Harderian gland in some terrestrial vertebrates may produce novel lipids that stabilize the tear film and protect against dry eye. Here, the nonpolar lipids in the Harderian gland and tears of the rabbit but absent in human tears were identified and tested in preclinical studies to determine whether they could treat severe EDED. Methods: Lipids were identified primarily by atmospheric pressure chemical ionization mass spectrometry (MS) and fragmentation MS/MS. An identified lipid was synthesized and formulated as an emulsion and as a cyclodextrin (CD) clathrate. Following doses with test agents and controls, tear film breakup time (TBUT), tear production, corneal fluorescein staining, macrophage infiltration, and goblet cell survival were measured using standard tests at 0, 2 and 4 weeks in an animal model of EDED. Results: The lipid emulsion increased TBUT (P < 0.01) and tear production (P < 0.05), while it decreased corneal staining (P < 0.01) compared to controls. The lipid CD formulation increased TBUT (P < 0.05) and tear production (P < 0.05) but had no significant effect on the remaining test parameters. There were no differences in macrophage infiltration and conjunctival impression cytology scores between the formulations and their vehicle controls. Conclusions: Lipids in the rabbit Harderian gland and tears differ from those identified in human meibum and tears. These unique rabbit lipids may confer a protective effect against EDED and, as supplements to human tears, fulfill a similar role.

Expression of Acyl-CoA wax-alcohol acyltransferase 2 (AWAT2) by human and rabbit meibomian glands and meibocytes

PURPOSE

Previously, we showed that Acyl-CoA wax-alcohol acyltransferase 2 (AWAT2), an essential enzyme required for meibum wax ester synthesis, was not expressed by immortalized human meibomian gland epithelial cells (hMGEC) in culture. To begin to understand the mechanisms controlling AWAT2 expression, we have analyzed its expression in human and rabbit meibomian glands and cultured meibocytes.

METHODS

Rabbit meibocyte progenitor cells (rMPC) were first grown in Cnt-BM.1 basal medium (Cellntec) supplemented with rhEGF, FGF10, and ROCK inhibitor (Y-27632 dihydrochloride), and then passed at 70-80% confluency with Accutase. Differentiation of rMPC to meibocytes (rMC) was induced by removal of Y-27632 and addition of 1 mM calcium with and without PPARγ agonists. RNA from the tissue, primary, passaged rMPC and differentiated rMC were obtained for AWAT2 qPCR analysis. Proteins and cells were evaluated for western blotting and neutral lipid synthesis, respectively. For comparison, human meibomian glands were separated for RNA and protein analysis. hMGEC was cultured to collect RNA and protein.

RESULTS

Rabbit rMPCs were successfully grown, passaged, and differentiated, showing a significant increase in lipid droplet accumulation. AWAT2 RNA was highly expressed in tissue but showed a -16.9 log2 fold decrease in primary and passaged rMPCs and was not induced by differentiation to rMC. By comparison, human meibomian glands showed high expression of AWAT2, and hMGEC expressed non-detectable levels of AWAT2 transcripts or protein.

CONCLUSIONS

AWAT2 expression is lost in cultured rMPC and rMC suggesting that cells in culture do not undergo complete meibocyte differentiation and require yet to be identified culture conditions.

International Harmonization of Nomenclature and Diagnostic Criteria (INHAND): Nonproliferative and Proliferative Lesions of the Rabbit

The INHAND (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions Project (www.toxpath.org/inhand.asp) is a joint initiative of the Societies of Toxicologic Pathology from Europe (ESTP), Great Britain (BSTP), Japan (JSTP) and North America (STP) to develop an internationally accepted nomenclature for proliferative and non-proliferative lesions in laboratory animals. The purpose of this publication is to provide a standardized nomenclature for classifying microscopic lesions observed in most tissues and organs from the laboratory rabbit used in nonclinical safety studies. Some of the lesions are illustrated by color photomicrographs. The standardized nomenclature presented in this document is also available electronically on the internet (http://www.goreni.org/). Sources of material included histopathology databases from government, academia, and industrial laboratories throughout the world. Content includes spontaneous lesions as well as lesions induced by exposure to test materials. Relevant infectious and parasitic lesions are included as well. A widely accepted and utilized international harmonization of nomenclature for lesions in laboratory animals will provide a common language among regulatory and scientific research organizations in different countries and increase and enrich international exchanges of information among toxicologists and pathologists.

Effects of Aging on Human Meibum

Purpose

The purpose of this study was to determine if aging affects meibum lipid composition in non-meibomian gland dysfunction (MGD)/non-dry eye (DE) population. Aging has been repeatedly linked to pathological changes in various tissues and organs, including the onset of MGD and DE, in a number of clinical and population-wide surveys. Both conditions have been associated with abnormal meibum secretion and composition, among other factors. However, the chemical basis for such a connection has not been established yet.

Methods

To identify and characterize possible changes in the meibum and meibogenesis with aging, lipidomic analyses of meibum samples collected from human subjects of two age groups - young (29 ± 5 years, n = 21) and elderly (68 ± 7 years, n = 29) - with similar male to female ratios in each group were conducted. Intact lipid species from major lipid groups of meibum (such as wax esters, cholesteryl esters, free cholesterol, triacylglycerols, etc.) were compared using lipidome-wide untargeted (such as Principal Component Analysis) and targeted (such as Orthogonal Projections to Latent Structures Discriminant Analysis) approaches, along with focused analyses of specific lipid species in liquid-chromatography mass spectrometry (LC-MS) and tandem mass spectrometry (MS-MS) experiments.

Results

Extremely high similarities of meibum lipids in the two age groups were observed, with only minor changes in the individual lipid species. The magnitude of the intergroup variability for tested lipid species was comparable to the intragroup variability for the same meibum components. No statistically significant differences in the lipid esterification, elongation, and unsaturation patterns were observed.

Conclusions

Chronological aging itself seems to have only minor effect on meibogenesis in healthy, non-MGD/non-DE subjects.

Lipidomic analysis of meibomian glands from type-1 diabetes mouse model and preliminary studies of potential mechanism

Diabetes is a significant risk factor for meibomian gland dysfunction (MGD), but its mechanism is poorly understood. The main function of the meibomian glands (MGs) is to synthesize, store, and secrete lipids. In this study, we found that the amount of lipids in the meibomian acini in STZ-induced type 1 diabetic mice decreased, and the lipid droplets became larger and irregular. In all, 31 lipid subclasses were identified in the mouse MGs, which contained 1378 lipid species in total through lipidomics analysis based on LC-MS/MS. Diabetes caused a significant increase in the content of ceramides (Cer) in the MGs but a significant decrease in the ration of sphingomyelin to ceramides (SM/Cer). The quantity of meibocytes in diabetic mice was dramatically decreased, and the proliferation activity was alleviated, which may be associated with cell cycle arrest caused by diabetes-induced abnormal Cer metabolism in MGs. We found an increase in macrophage and neutrophils infiltration in the diabetic MGs, which may be related to the significant reduction of AcCa in diabetic MGs. Taken together, the results of the present study demonstrated that diabetes induced disruption of lipid homeostasis in MGs, which may mediate the decreased cell proliferation and increased inflammation caused by diabetes in MGs.

Meibomian Gland Dysfunction in Ocular Graft vs. Host Disease: A Need for Pre-Clinical Models and Deeper Insights

Despite decades of experience with hematopoietic stem cell transplantation, we are still faced with the delicate equipoise of achieving stable ocular health post-transplantation. This is because ocular graft-versus-host disease (oGvHD) following hematopoietic stem cell transplantation frequently occurs (≥50%) among transplant patients. To date, our understanding of the pathophysiology of oGvHD especially the involvement of the meibomian gland is still limited as a result of a lack of suitable preclinical models among other. Herein, the current state of the etiology and, pathophysiology of oGvHD based on existing pre-clinical models are reviewed. The need for additional pre-clinical models and knowledge about the involvement of the meibomian glands in oGvHD are emphasized.

Dexmedetomidine and Tear Production: Evaluation in Dogs as Spontaneous Model for Ocular Surface Disorders

Simple Summary

The general anesthesia or sedation reduces both the tear production and the stability of tear film that protect corneal surface, predisposing itself to the exposure keratopathy. The aim of the present study was to evaluate the effects of intramuscular dexmedetomidine (DEX) on canine tear production, measured by standardized Schirmer Tear Test 1 (STT-1) strips, for the 8 h following sedation, in dogs. A significant effect of time on canine tear production was found, highlighting that dexmedetomidine sedative protocol significantly affects tear production in dogs. It is recommended to treat the canine eyes with tear substitutes to protect ocular surface health and the welfare of the dogs. The ocular lubrication should be performed during and up to 12 h after sedation. The present report could provide preliminary information to better understand the effect of DEX on the tear film dynamics.

Abstract

Tear film provides lubrication and protection to the ocular surface. The sedation reduces tear production, often leading to perioperative exposure keratopathy. The aim of the present study was to report the effects of intramuscular dexmedetomidine on canine tear production, measured by STT-1, for an experimental period of 8 h after sedation. Ten dogs who underwent sedation for routine radiologic assessment were recruited for the study. In all animals, tear production in right and left eyes was measured 15 min before sedation (T0: basal values) and 20 min (T20), 1 h (T1), 2 h (T2), 4 h (T4) and 8 h (T8) after drug administration. Analysis of variance and post hoc Bonferroni test (p < 0.05) were performed. A significant effect of time on canine tear production was found. The tear production returned to basal values at T8. So, it is recommended to treat the canine eyes with tear substitutes during and up to 12 h after sedation.

Lipid conformational order and the etiology of cataract and dry eye

Lens and tear film lipids are as unique as the systems they reside in. The major lipid of the human lens is dihydrosphingomylein, found in quantity only in the lens. The lens contains a cholesterol to phospholipid molar ratio as high as 10:1, more than anywhere else in the body. Lens lipids contribute to maintaining lens clarity, and alterations in lens lipid composition due to age are likely to contribute to cataract. Lens lipid composition reflects adaptations to the unique characteristics of the lens: no turnover of lens lipids or proteins; the lowest amount of oxygen of any tissue; and contains almost no intracellular organelles. The tear film lipid layer (TFLL) is also unique. The TFLL is a thin (100 nm) layer of lipid on the surface of tears covering the cornea that contributes to tear film stability. The major lipids of the TFLL are wax esters and cholesterol esters that are not found in the lens. The hydrocarbon chains associated with the esters are longer than those found anywhere else in the body (as long as 32 carbons), and many are branched. Changes in the composition and structure of the 30,000 different moieties of TFLL contribute to the instability of tears. The focus of the current review is how spectroscopy has been used to elucidate the relationships between lipid composition, conformational order and function, and the etiology of cataract and dry eye.

Depletion of Cholesteryl Esters Causes Meibomian Gland Dysfunction-Like Symptoms in a Soat1-Null Mouse Model

Previous studies on ablation of several key genes of meibogenesis related to fatty acid elongation, omega oxidation, and esterification into wax esters have demonstrated that inactivation of any of them led to predicted changes in the meibum lipid profiles and caused severe abnormalities in the ocular surface and Meibomian gland (MG) physiology and morphology. In this study, we evaluated the effects of Soat1 ablation that were expected to cause depletion of the second largest class of Meibomian lipids (ML)-cholesteryl esters (CE)-in a mouse model. ML of the Soat1-null mice were examined using liquid chromatography high-resolution mass spectrometry and compared with those of Soat1+/- and wild-type mice. Complete suppression of CE biosynthesis and simultaneous accumulation of free cholesterol (Chl) were observed in Soat1-null mice, while Soat1+/- mutants had normal Chl and CE profiles. The total arrest of the CE biosynthesis in response to Soat1 ablation transformed Chl into the dominant lipid in meibum accounting for at least 30% of all ML. The Soat1-null mice had clear manifestations of dry eye and MG dysfunction. Enrichment of meibum with Chl and depletion of CE caused plugging of MG orifices, increased meibum rigidity and melting temperature, and led to a massive accumulation of lipid deposits around the eyes of Soat1-null mice. These findings illustrate the role of Soat1/SOAT1 in the lipid homeostasis and pathophysiology of MG.

Establishment of a Tear Ferning Test Protocol in the Mouse Model

Purpose

Analysis of ferning formation after tear drop desiccation on a glass slide has been applied as a simple method to examine tear normality and is referred to as the tear ferning (TF) test. Despite use of the TF test in clinical settings and in some animals, thus far no TF test protocol has been developed for the mouse model. This study aimed to establish a mouse TF test protocol that can be used for dry eye research using the mouse as the study model.

Methods

Tear samples were collected from 24 healthy mice after repeated flushes with 2, 5, 10, or 20 µL wash solutions, either 0.9% NaCl saline or sterile water, on the ocular surface. After sample collection, TF tests were performed at variable drop volumes (2–20 µL), at a relative humidity of either 46% ± 2% or 53% ± 2%, and with temperature fixed at 24°C ± 2°C for comparison. Moreover, the influence of osmolarity (between 280 and 360 mOsm/L) and pH values (6.5–8.0) and the effect of centrifugation (4000 rpm, 10 minutes) on ferning formation were examined. Reproducibility and ferning storage stability were also determined.

Results

An optimized protocol was established with relative humidity at 46% ± 2% and drop aliquot at 2 µL, using 0.9% NaCl saline as the wash solution. Using sterilized water as the wash solution did not result in any crystalloid formation. Centrifugation did not aid ferning formation in any of the samples. Higher osmolarity increased ferning formation from grades between 0 to 1 to grades between 2 to 3, but pH values that varied between 6.5 and 8.0 did not affect ferning formation. The established mouse TF test protocol also displayed reproducibility and storage stability.

Conclusions

A TF test protocol for the mouse model was established that could be used for comparative analyses under various ocular surface disease conditions.

Translational Relevance

This mouse TF test protocol will facilitate the application of basic research into the mouse model to clinical care.

Lacritin proteoforms prevent tear film collapse and maintain epithelial homeostasis

Lipids in complex, protein-enriched films at air/liquid interfaces reduce surface tension. In the absence of this benefit, the light refracting and immunoprotective tear film on eyes would collapse. Premature collapse, coupled with chronic inflammation compromising visual acuity, is a hallmark of dry eye disease affecting 7 to 10% of individuals worldwide. Although collapse seems independent of mutation (unlike newborn lung alveoli), selective proteome and possible lipidome changes have been noted. These include elevated tissue transglutaminase and consequent inactivation through C-terminal cross-linking of the tear mitogen lacritin, leading to significant loss of lacritin monomer. Lacritin monomer restores homeostasis via autophagy and mitochondrial fusion and promotes basal tearing. Here, we discover that lacritin monomer C-terminal processing, inclusive of cysteine, serine, and metalloproteinase activity, generates cationic amphipathic α-helical proteoforms. Such proteoforms (using synthetic peptide surrogates) act like alveolar surfactant proteins to rapidly bind and stabilize the tear lipid layer. Immunodepletion of C- but not N-terminal proteoforms nor intact lacritin, from normal human tears promotes loss of stability akin to human dry eye tears. Stability of these and dry eye tears is rescuable with C- but not N-terminal proteoforms. Repeated topical application in rabbits reveals a proteoform turnover time of 7 to 33 h with gradual loss from human tear lipid that retains bioactivity without further processing. Thus, the processed C-terminus of lacritin that is deficient or absent in dry eye tears appears to play a key role in preventing tear film collapse and as a natural slow release mechanism that restores epithelial homeostasis.

Meibomian Gland Dysfunction: What Have Animal Models Taught Us?

Studies have estimated that currently 344 million people worldwide and 16.4 million adults in the US have some form of dry eye disease (DED). It is believed that approximately 70% of DED cases are due to some form of evaporative dry eye, for which Meibomian gland dysfunction (MGD) is the major cause. Unfortunately, currently there is no effective treatment for MGD, and solely palliative care is available. Given the importance of MGD in DED, there has been a growing interest in studying Meibomian gland development, homeostasis and pathology, and, also, in developing therapies for treating and/or preventing MGD. For such, animal models have shown to be a vital tool. Much of what is known today about the Meibomian gland and MGD was learnt from these important animal models. In particular, canine and rabbit models have been essential for studying the physiopathology and progression of DED, and the mouse model, which includes different knockout strains, has enabled the identification of specific pathways potentially involved in MGD. Herein, we provide a bibliographic review on the various animal models that have been used to study Meibomian gland development, Meibomian gland homeostasis and MGD, primarily focusing on publications between 2000 and 2020.

Obese Mice with Dyslipidemia Exhibit Meibomian Gland Hypertrophy and Alterations in Meibum Composition and Aqueous Tear Production

BACKGROUND

Dyslipidemia may be linked to meibomian gland dysfunction (MGD) and altered meibum lipid composition. The purpose was to determine if plasma and meibum cholesteryl esters (CE), triglycerides (TG), ceramides (Cer) and sphingomyelins (SM) change in a mouse model of diet-induced obesity where mice develop dyslipidemia.

METHODS

Male C57/BL6 mice (8/group, age = 6 wks) were fed a normal (ND; 15% kcal fat) or an obesogenic high-fat diet (HFD; 42% kcal fat) for 10 wks. Tear production was measured and meibography was performed. Body and epididymal adipose tissue (eAT) weights were determined. Nano-ESI-MS/MS and LC-ESI-MS/MS were used to detect CE, TG, Cer and SM species. Data were analyzed by principal component analysis, Pearson's correlation and unpaired t-tests adjusted for multiple comparisons; significance set at p ≤ 0.05.

RESULTS

Compared to ND mice, HFD mice gained more weight and showed heavier eAT and dyslipidemia with higher levels of plasma CE, TG, Cer and SM. HFD mice had hypertrophic meibomian glands, increased levels of lipid species acylated by saturated fatty acids in plasma and meibum and excessive tear production.

CONCLUSIONS

The majority of meibum lipid species with saturated fatty acids increased with HFD feeding with evidence of meibomian gland hypertrophy and excessive tearing. The dyslipidemia is associated with altered meibum composition, a key feature of MGD.

Deficiency in Acyl-CoA:Wax Alcohol Acyltransferase 2 causes evaporative dry eye disease by abolishing biosynthesis of wax esters

Lipids secreted by the meibomian glands (MGs) of the eyelids are essential to the protection of the eye's surface. An altered meibum composition represents the primary cause of evaporative dry eye disease (DED). Despite the critical importance of the meibum, its biosynthetic pathways and the roles of individual lipid components remain understudied. Here, we report that the genetic deletion of Acyl-CoA:wax alcohol acyltransferase 2 (AWAT2) causes the obstruction of MGs and symptoms of evaporative DED in mice. The lipid composition of the meibum isolated from Awat2^-/- mice revealed the absence of wax esters, which was accompanied by a compensatory overproduction of cholesteryl esters. The resulting increased viscosity of meibum led to the dilation of the meibomian ducts, and the progressive degeneration of the MGs. Overall, we provide evidence for the main physiological role of AWAT2 and establish Awat2^-/- mice as a model for DED syndrome that can be used in studies on tear film-oriented therapies.

Comprehensive profiling of Asian and Caucasian meibomian gland secretions reveals similar lipidomic signatures regardless of ethnicity

Meibum-a lipid secretion that is produced by Meibomian glands (MG) in a process termed meibogenesis-plays a critical role in ocular surface physiology. Abnormalities in the chemical composition of meibum were linked to widespread ocular pathologies-dry eye syndrome (DES) and MG dysfunction (MGD). Importantly, in epidemiologic studies the Asian population was shown to be prone to these pathologies more than the Caucasian one, which was tied to differences in their meibomian lipids. However, biochemical data to support these observations and conclusions are limited. To determine if non-DES/non-MGD Asian meibum was significantly different from that of Caucasians, individual samples of meibum collected from ethnic Asian population living in Japan were compared with those of Caucasians living in the USA. These experiments revealed that composition of major lipid classes, such as wax esters (WE), cholesteryl esters (CE), triacylglycerols, (O)-acylated ω-hydroxy fatty acids (OAHFA), cholesteryl sulfate, cholesteryl esters of OAHFA, and diacylated α,ω-dihydroxy fatty alcohols remained invariable in both races, barring a minor (< 10%; p < 0.01) increase in the Asian CE/WE ratio. Considering the natural variability range for most meibomian lipids (app. ± 15% of the Mean), these differences in meibogenesis were deemed to be minimal and unlikely to have a measurable physiological impact.

An eye on the dog as the scientist's best friend for translational research in ophthalmology: Focus on the ocular surface

Preclinical animal studies provide valuable opportunities to better understand human diseases and contribute to major advances in medicine. This review provides a comprehensive overview of ocular parameters in humans and selected animals, with a focus on the ocular surface, detailing species differences in ocular surface anatomy, physiology, tear film dynamics and tear film composition. We describe major pitfalls that tremendously limit the translational potential of traditional laboratory animals (i.e., rabbits, mice, and rats) in ophthalmic research, and highlight the benefits of integrating companion dogs with clinical analogues to human diseases into preclinical pharmacology studies. This One Health approach can help accelerate and improve the framework in which ophthalmic research is translated to the human clinic. Studies can be conducted in canine subjects with naturally occurring or noninvasively induced ocular surface disorders (e.g., dry eye disease, conjunctivitis), reviewed herein, and tear fluid can be easily retrieved from canine eyes for various bioanalytical purposes. In this review, we discuss common tear collection methods, including capillary tubes and Schirmer tear strips, and provide guidelines for tear sampling and extraction to improve the reliability of analyte quantification (drugs, proteins, others).

Lipid polarity gradient formed by ω-hydroxy lipids in tear film prevents dry eye disease

Meibum lipids form a lipid layer on the outermost side of the tear film and function to prevent water evaporation and reduce surface tension. (O-Acyl)-ω-hydroxy fatty acids (OAHFAs), a subclass of these lipids, are thought to be involved in connecting the lipid and aqueous layers in tears, although their actual function and synthesis pathway have to date remained unclear. Here, we reveal that the fatty acid ω-hydroxylase Cyp4f39 is involved in OAHFA production. Cyp4f39-deficient mice exhibited damaged corneal epithelium and shortening of tear film break-up time, both indicative of dry eye disease. In addition, tears accumulated on the lower eyelid side, indicating increased tear surface tension. In Cyp4f39-deficient mice, the production of wax diesters (type 1ω and 2ω) and cholesteryl OAHFAs was also impaired. These OAHFA derivatives show intermediate polarity among meibum lipids, suggesting that OAHFAs and their derivatives contribute to lipid polarity gradient formation for tear film stabilization.

Comparative Transcriptomic and Lipidomic Analyses of Human Male and Female Meibomian Glands Reveal Common Signature Genes of Meibogenesis

Meibum is a lipid secretion that is produced by holocrine Meibomian glands (MGs). MGs are a specialized type of sebaceous glands that are embedded in the human eyelids. Chemically, meibum and sebum are different. A detailed characterization of lipidome and transcriptome of MG is required to deconvolute a complex and poorly characterized array of biosynthetic reactions (termed meibogenesis) that lead to formation of meibum. Changes in the composition and quality of meibum have been linked to various ocular disorders, some of which are more prevalent in males, while others in females. To establish the role of gender in meibogenesis in humans, we characterized MG transcriptomes and lipidomes of females and males, and identified signature genes of meibogenesis in both genders. Specimens of MG tissues were subjected to mRNA microarray analyses. Chemical composition of meibum samples was assessed chromatographically and mass spectrometrically. Both targeted and untargeted approaches were used. About 290 signature genes of meibogenesis were identified. The analyses of their expression patterns demonstrated no major differences between the genders. Lipid profiling of major classes of meibomian lipids, such as wax esters, cholesteryl esters, free cholesterol, (O)-acylated omega-hydroxy fatty acids (OAHFA), cholesteryl esters of OAHFA, and triacylglycerols, also demonstrated only minor (and random) differences in these lipids. The results of transcriptomic analyses correlated well with lipidomic data. Taken together, our data imply that in males and females, meibogenesis proceeds in a similar fashion, yielding secretions with similar, highly conserved, compositions. This finding is important for designing novel, gender-independent diagnostic and therapeutic approaches to various MG-related diseases and pathological conditions.

Membrane-associated mucins of the ocular surface: New genes, new protein functions and new biological roles in human and mouse

The mucosal glycocalyx of the ocular surface constitutes the point of interaction between the tear film and the apical epithelial cells. Membrane-associated mucins (MAMs) are the defining molecules of the glycocalyx in all mucosal epithelia. Long recognized for their biophysical properties of hydration, lubrication, anti-adhesion and repulsion, MAMs maintain the wet ocular surface, lubricate the blink, stabilize the tear film and create a physical barrier to the outside world. However, it is increasingly appreciated that MAMs also function as cell surface receptors that transduce information from the outside to the inside of the cell. A number of excellent review articles have provided perspective on the field as it has progressed since 1987, when molecular cloning of the first MAM was reported. The current article provides an update for the ocular surface, placing it into the broad context of findings made in other organ systems, and including new genes, new protein functions and new biological roles. We discuss the epithelial tissue-equivalent with mucosal differentiation, the key model system making these advances possible. In addition, we make the first systematic comparison of MAMs in human and mouse, establishing the basis for using knockout mice for investigations with the complexity of an in vivo system. Lastly, we discuss findings from human genetics/genomics, which are providing clues to new MAM roles previously unimagined. Taken together, this information allows us to generate hypotheses for the next stage of investigation to expand our knowledge of MAM function in intracellular signaling and roles unique to the ocular surface.

Dyslipidemia and Meibomian Gland Dysfunction: Utility of Lipidomics and Experimental Prospects with a Diet-Induced Obesity Mouse Model

Meibomian gland dysfunction (MGD) is the leading cause of dry eye disease and loss of ocular surface homeostasis. Increasingly, several observational clinical studies suggest that dyslipidemia (elevated blood cholesterol, triglyceride or lipoprotein levels) can initiate the development of MGD. However, conclusive evidence is lacking, and an experimental approach using a suitable model is necessary to interrogate the relationship between dyslipidemia and MGD. This systematic review discusses current knowledge on the associations between dyslipidemia and MGD. We briefly introduce a diet-induced obesity model where mice develop dyslipidemia, which can serve as a potential tool for investigating the effects of dyslipidemia on the meibomian gland. Finally, the utility of lipidomics to examine the link between dyslipidemia and MGD is considered.

On the pivotal role of Elovl3/ELOVL3 in meibogenesis and ocular physiology of mice

The purpose of this study was to examine the role of Elovl3 gene in meibogenesis and the impact of ELOVL3 protein ablation on the physiology of the mouse ocular surface and Meibomian glands (MGs). Elovl3 knockout, ELOVL3-ablated (E3hom) mice and their wild type littermates (E3wt) were studied side by side. E3hom mice had abnormal ocular phenotypes such as delayed eye opening, weeping eyes, crusty eyelids, eyelid edema, highly vascularized cornea and tarsal plates (TPs), slit eye, and increased tearing that resemble symptoms observed in human subjects with various forms of dry eye, MG dysfunction and blepharitis. Lipid profiling of E3hom TPs was conducted using chromatography and mass spectrometry. The analyses revealed that the lipid composition of E3hom TPs was strikingly different from that of their E3wt littermates. The mutation affected major classes of meibomian lipids - cholesteryl esters, wax esters, and cholesteryl esters of (O)-acylated w-hydroxy fatty acids. The studies illuminated the central role of ELOVL3 in producing C_21:0-C_29:0 fatty acids, including odd-chain and branched ones. Ablation of ELOVL3 leads to selective changes in the lipid composition of meibum, making E3hom mice instrumental in studying the mechanisms of the biosynthesis of meibum and modeling various pathologies of human ocular surface and adnexa.-Butovich, I. A., Wilkerson, A., Bhat, N., McMahon, A., Yuksel, S. On the pivotal role of Elovl3/ELOVL3 in meibogenesis and ocular physiology of mice.

Effects of sex (or lack thereof) on meibogenesis in mice (Mus musculus): Comparative evaluation of lipidomes and transcriptomes of male and female tarsal plates

The possible role of sex in the biosynthesis of lipids in the Meibomian glands (termed meibogenesis) remains unclear. To determine if there were any major sex-specific differences in the lipid composition of meibomian gland secretions (meibum) and gene expression patterns (GEP) related to meibogenesis, we conducted a study using healthy, age and diet-matched young adult wild-type C57BL/6J mice (2-2.5 month old). Tarsal plates (TP) were surgically excised from the eyelids of mice and subjected to transcriptomic and lipidomic analyses. The GEP were studied using mRNA microarrays. Lipids were extracted with organic solvents and analyzed using liquid chromatography and mass spectrometry. GEP in the TP of female and male mice demonstrated no statistically significant differences in the expression levels of the main protein-coding genes related to lipid metabolism and storage in general, and meibogenesis specifically (such as Elovl, Scd, Fads, Soat, Far, Awat, Acat, Lss, Dhcr, Hmgcr, Hmgcs, Dgat, Bckdh, Dbt, Fasn, and Plin, among others). The meibomian lipid profiles of female and male mice were virtually indistinguishable: all major lipids such as waxes, cholesteryl esters, cholesterol, (O)-acylated omega-hydroxy fatty acids (OAHFA), cholesteryl esters of OAHFA etc., were present in similar ratios. It seems that the major biosynthetic pathways in the Meibomian glands of male and female mice function in a similar fashion and produce secretions of the same overall chemical composition.

Untargeted lipidomic analysis of human tears: A new approach for quantification of O-acyl-omega hydroxy fatty acids

PURPOSE

The purpose of this work was to optimize methodology to analyze the human tear film lipids by using untargeted, direct infusion electrospray ionization-mass spectrometry (ESI-MS) to establish the analytical approach for a large-scale clinical translational study of tear film lipids in ocular surface disease, particularly associated with the O-acyl-omega hydroxy fatty acids (OAHFAs).

METHODS

Meibum and tear samples were collected from both eyes of five normal subjects without ocular disease using two different microcapillary collection tubes, glass and polytetrafluoroethylene (PTFE). An untargeted lipidomics approach was used to analyze the lipids in human tear and meibum samples using direct infusion ESI-MS in positive and negative ion modes. Direct and indirect quantification methods were evaluated.

RESULTS

The amount of OAHFAs measured in tears using these techniques was approximately 0.7-0.8% of the total lipids. More phospholipids, including phosphatidylcholine and sphingomyelin, were detected in the tear samples associated with glass microcapillaries compared to PTFE.

CONCLUSIONS

Reliable assessment of lipids in small volumes of tear film is possible using high resolution, untargeted ESI-MS in positive and negative ion modes. Using this technique, the concentration of OAHFAs can be quantified, as can the presence of other polar lipids.

Lipidomic analysis of meibomian gland secretions from the tree shrew: Identification of candidate tear lipids critical for reducing evaporation

Lipids secreted from the meibomian glands form the outermost layer of the tear film and reduce its evaporation. Abnormal changes in the quantities or compositions of lipids present in meibomian gland secretions (meibum) are known to lead to dry eye disease, although the underlying mechanism is not yet well understood. The tree shrew is the non-primate mammal most closely related to humans. To assess the utility of the tree shrew as a model for the study of dry eye disease, we analyzed the lipid profile of tree shrew meibum using an untargeted ESI-MS and MS/MS^all shotgun approach. The resulting lipidome shared many similarities with human meibum, while displaying some interesting differences. For example, several classes of lipids, including wax esters, cholesteryl esters, diesters, and (O-acyl)-ω-hydroxy fatty acids, had relatively longer chain lengths in tree shrew meibum. These increases in length may promote more effective reduction of tear evaporation in the tree shrew, which likely underlies the much longer blinking interval of this mammal. Our results suggest that the tree shrew could be an effective model for the study of dry eye.

Comprehensive shotgun lipidomics of human meibomian gland secretions using MS/MSall with successive switching between acquisition polarity modes

The lipid composition of human meibomian gland secretions (meibum) has been analyzed using both targeted and untargeted mass spectrometric approaches, each of which has its advantages and disadvantages. Herein we report the results of shotgun lipidomic profiling of human meibum using a new approach that combines the advantages of targeted and untargeted analyses to yield highly sensitive and comprehensive profiles. Samples containing an estimated 7-13 µg (8-16 nL) of human meibum lipids were analyzed using MS/MS^all, an untargeted approach for MS/MS. Using MS/MS^all with ESI and successive polarity switching, we obtained tandem mass spectra in both modes at every 1 Da step for all ions in the m/z 200-1,200 range. In approximately 12 min, a total of 2 MS spectra and 2,000 MS/MS spectra were acquired for each sample, from which targeted analysis information was extracted. This approach allowed for the comprehensive and highly sensitive detection of meibum lipids, including species low in abundance. Altogether, more than 600 unique lipid molecular species were identified in meibum, 3 times more than previously reported in untargeted analyses of meibum samples. This untargeted MS and MS/MS^all approach may be extended to other biological systems for the detection of lipids with sensitivity comparable to targeted analysis.

Targeted Ocular Drug Delivery with Pharmacokinetic/Pharmacodynamic Considerations

The development of ophthalmic drug delivery systems is a long and comprehensive process including research, nonclinical, and clinical development stages. It is critical to understand the similarity and differences between animal models and patients. There are many anatomically and physiologically important parameters for targeted drug delivery into eyes. This paper reviews the constraints to various routes of ocular drug delivery and discusses the respective pharmacokinetic considerations, to lay the foundation for formulation approaches pharmaceutical scientists can use to maximize successful drug delivery for each route. The overall goal is to give both researchers and drug developers a better understanding of ocular drug delivery and offer tools to successfully develop new medicines that will fulfil unmet medical needs and improve patients' quality of life.

Evaluation of lacrimal production, osmolarity, crystallization, proteomic profile, and biochemistry of capuchin monkeys' tear film

BACKGROUND

To evaluate the composition and characteristics of capuchin monkey (CM) tear film.

METHODS

Eleven CM (Sapajus sp.) was evaluated. Strip meniscometry test (SMT), osmolarity, and tear ferning test (TFT) (by Rolando and Masmali scales) were assessed. Tear protein profile (SDS-PAGE), and total protein, albumin, urea, glucose, and cholesterol levels in tear film and blood serum were screened.

RESULTS

Median ± semi-interquatil range for SMT and osmolarity values were 8.0 ± 1.625 and 303.0 ± 9.875, respectively. TFT for Rolando was 2.0 ± 0.5, and Masmali was 2.0 ± 0.0. Monkeys's tear obtained type II and III for Rolando, and 1 and 2 for Masmali. Tear components showed protein bands among 23-217 kDa, and presence of albumin, urea, glucose, and cholesterol.

CONCLUSIONS

The results of SMT, osmolarity, TFT, SDS-PAGE, and tear biochemistry may serve as a reference baseline for CM, and the data may serve as a basis for future experimental model evaluations.

Mass spectrometry-directed structure elucidation and total synthesis of ultra-long chain (O-acyl)-ω-hydroxy fatty acids

The (O-acyl)-ω-hydroxy FAs (OAHFAs) comprise an unusual lipid subclass present in the skin, vernix caseosa, and meibomian gland secretions. Although they are structurally related to the general class of FA esters of hydroxy FAs (FAHFAs), the ultra-long chain (30-34 carbons) and the putative ω-substitution of the backbone hydroxy FA suggest that OAHFAs have unique biochemistry. Complete structural elucidation of OAHFAs has been challenging because of their low abundance within complex lipid matrices. Furthermore, because these compounds occur as a mixture of closely related isomers, insufficient spectroscopic data have been obtained to guide structure confirmation by total synthesis. Here, we describe the full molecular structure of ultra-long chain OAHFAs extracted from human meibum by exploiting the gas-phase purification of lipids through multi-stage MS and novel multidimensional ion activation methods. The analysis elucidated sites of unsaturation, the stereochemical configuration of carbon-carbon double bonds, and ester linkage regiochemistry. Such isomer-resolved MS guided the first total synthesis of an ultra-long chain OAHFA, which, in turn, confirmed the structure of the most abundant OAHFA found in human meibum, OAHFA 50:2. The availability of a synthetic OAHFA opens new territory for future investigations into the unique biophysical and biochemical properties of these lipids.

Meibomian Glands and Ocular Surface Changes After Closure of Meibomian Gland Orifices in Rabbits

PURPOSE

To investigate changes in the corneal surface, tear film, and meibomian glands after meibomian gland orifice closure.

METHODS

Seventy-two eyes of 36 rabbits were used. In the study group (n = 36), the meibomian gland orifices of both upper and lower eyelids in the right eyes were electrosurgically coagulated. The 36 untreated left eyes were used as controls. Corneal wetting properties were measured 1, 3, 7, and 14 days after coagulation. The eyelid sections were stained with anti-cytokeratin (CK)1, CK5, and CK6 antibody 8 weeks after coagulation. The area of the secretory acini around one meibomian gland duct was measured, and meibography of rabbits was performed 8 weeks after meibomian gland orifice closure.

RESULTS

Three days after coagulation, the corneal wetting property was decreased compared with controls. The meibomian gland ducts gradually dilated in the study group over time. The epithelium of the central ducts in both groups was stained with CK5 and CK6, but not CK1. Although the mean area of the secretory acini in the study group (0.10 ± 0.04 mm) was significantly smaller than that of the control group (0.18 ± 0.04; P = 0.004), meibography showed normal morphology in both study and control groups.

CONCLUSIONS

Meibomian gland orifice closure reduced corneal wetting property and induced meibomian gland duct dilation accompanied by shrinkage of secretory acini. Meibography could not detect early changes in the meibomian gland after closure of the orifice. Therefore, when the orifice is obstructed, more active treatments are needed before structural changes occur.

Meibomian glands, meibum, and meibogenesis

Meibum is a lipid-rich secretion that is produced by fully differentiated meibocytes in the holocrine Meibomian glands (MG) of humans and most mammals. The secretion is a part of a defense mechanism that protects the ocular surface from hazardous environmental factors, and from desiccation. Meibomian lipids that have been identified in meibum are very diverse and unique in nature. The lipid composition of meibum is different from virtually any other lipid pool found in the human body. In fact, meibum is quite different from sebum, which is the closest secretion that is produced by anatomically, physiologically, and biochemically related sebaceous glands. However, meibum of mice have been shown to closely resemble that of humans, implying similar biosynthetic mechanisms in MG of both species. By analyzing available genomic, immunohistochemical, and lipidomic data, we have envisioned a unifying network of enzymatic reactions that are responsible for biosynthesis of meibum, which we call meibogenesis. Our current theory is based on an assumption that most of the biosynthetic reactions of meibogenesis are catalyzed by known enzymes. However, the main features that make meibum unique - the ratio of identified classes of lipids, the extreme length of its components, extensive ω-hydroxylation of fatty acids and alcohols, iso- and anteiso-branching of meibomian lipids (e.g. waxes), and the presence of rather unique complex lipids with several ester bonds - make it possible that either the activity of known enzymes is altered in MG, or some unknown enzymes contribute to the processes of meibogenesis, or both. Studies are in progress to elucidate meibogenesis on molecular level.

TFOS DEWS II Tear Film Report

The members of the Tear Film Subcommittee reviewed the role of the tear film in dry eye disease (DED). The Subcommittee reviewed biophysical and biochemical aspects of tears and how these change in DED. Clinically, DED is characterized by loss of tear volume, more rapid breakup of the tear film and increased evaporation of tears from the ocular surface. The tear film is composed of many substances including lipids, proteins, mucins and electrolytes. All of these contribute to the integrity of the tear film but exactly how they interact is still an area of active research. Tear film osmolarity increases in DED. Changes to other components such as proteins and mucins can be used as biomarkers for DED. The Subcommittee recommended areas for future research to advance our understanding of the tear film and how this changes with DED. The final report was written after review by all Subcommittee members and the entire TFOS DEWS II membership.